Liquid crystal display devices have excellent characteristics such as small thicknesses, light weights and low power-consumption among various display devices, and therefore are widely used in image displaying devices such as television sets and VCRs and office automation equipment such as monitors, word processors and personal computers.
Twisted nematic (TN) mode liquid crystal display devices using nematic liquid crystal, for example, have been previously put into practical use. A TN mode liquid crystal display device includes a pair of opposed substrates and a liquid crystal layer provided between the substrates, and generates an electric field between electrodes provided on the respective substrate, thereby driving the liquid crystal layer. Specifically, switching is performed such that the tilt angle of liquid crystal molecules in the liquid crystal layer with respect to one of the substrates increases based on whether an electric field is present or not between the substrates. However, the TN mode liquid crystal display device has drawbacks such as a low response speed and a narrow viewing angle.
On the other hand, as a display mode having a high response speed and a wide viewing angle, ferroelectric liquid crystal (FLC), anti-ferroelectric liquid crystal (AFLC) and others are known. However, these crystals have great deficiencies in shock resistance and temperature characteristics, and thus have not been widely used yet.
In view of this, an in-phase switching (IPS) mode in which switching is performed to have liquid crystal molecules always parallel to a substrate is known to date (see, for example, Patent literature 1.) For example, as shown in FIG. 20, a liquid crystal display device 100 in the IPS mode includes: a pair of substrates sandwiching a liquid crystal layer; first and second electrodes 101 and 102 serving as a pair of parallel electrodes placed on the surface of one of the substrates.
The liquid crystal display device 100 further includes: a plurality of parallel scanning lines 103 running in parallel with each other on one of the substrates; a plurality of signal lines 104 orthogonal to the scanning lines 103; common lines 105 parallel to the scanning lines 103; and thin-film transistors (hereinafter, referred to as TFTs) 106 provided in respective pixels defined by the scanning lines 103 and the signal lines 104.
In each of the pixels, the TFT 106 is connected to one of the signal lines 104 and the base end of the first electrode 101. The front end portion of the first electrode 101 extends in parallel with the signal lines 104. On the other hand, the common line 105 is connected to the base end of the second electrode 102. The front end portion of the second electrode 102 also extends in parallel with the signal lines 104. That is, the front end portions of the first and second electrodes 101 and 102 are parallel to each other.
An electric field is generated between the first and second electrodes 101 and 102 so that liquid crystal molecules in the liquid crystal layer are driven to be switched within a plane parallel to the substrates, thereby providing display. In this liquid crystal display device in the IPS mode, the tilt angle of liquid crystal molecules does not increase, thus obtaining a wider viewing angle than that of a device in the TN mode.
A liquid crystal display device in an FLC mode including a comb-like electrode dividing a pixel into two regions on a substrate is also known to date (see, for example, Non-patent literature 1.) As shown in FIGS. 21 and 22, which are perspective views schematically showing one pixel, a liquid crystal display device 100 disclosed in Non-patent literature 1 includes: an array substrate 121; a counter substrate 122 opposing the array substrate 121; and an FLC mode liquid crystal layer 123 provided between the array substrate 121 and the counter substrate 122.
A first electrode 101 and a second electrode 102 each having a comb-like electrode structure are formed on the array substrate 121. Each pixel is divided into two regions (i.e., domains) by the first and second electrodes 101 and 102. Liquid crystal molecules 108 in the liquid crystal layer 123 are driven by switching the intensity of an electric field generated between the first and second electrodes 101 and 102 to zero or a given value.
In a case where the intensity of the electric field is zero, as shown in FIG. 21, the liquid crystal layer 123 serves as a smectic liquid crystal layer in which liquid crystal molecules 108 are tilted at a given angle to the array substrate 121 and are oriented in spiral forms around axes L in the direction normal to the substrate 121 as an initial orientation.
On the other hand, in a case where an electric field with a given intensity is generated between the first and second electrodes 101 and 102, as shown in FIG. 22, liquid crystal molecules 108 in each layer rotate about the axes L with their tilt angle to the substrate increased and are aligned in a given direction in each region. This structure is formed to increase the display response speed of the liquid crystal display device 100.
However, in the liquid crystal display device 100 disclosed in Non-patent literature 1, when the intensity of the electric field between the electrodes 101 and 102 is zero, no refractive-index anisotropy occurs in the direction normal to the substrates 121 and 122. FIG. 23 is a graph for explaining an index ellipsoid. In FIG. 23, the direction X-Y indicates a direction parallel to the substrates 121 and 122 and the direction Z indicates a direction normal to the substrates 121 and 122. As shown in FIG. 23, out of three principal axes na, nb and nc for representing an index ellipsoid, the principal axis nc having the largest value is in the direction Z normal to the substrates when the intensity of an electric field is zero. When an electric field has a given intensity value, the principal axis nc rotates about the electric-field direction parallel to the substrate, as indicated by the arrow B.
Accordingly, the display characteristics of the liquid crystal display device 100 disclosed in Non-patent literature 1 can be assumed to be equivalent to those in a birefringence mode of a liquid crystal display device in which liquid crystal molecules are oriented vertically with respect to a substrate (hereinafter, simply referred to as a birefringence mode with a vertical orientation.) In addition, a pixel in the liquid crystal display device 100 has two domains in which the directions of respective electric fields are opposite to each other, so that the display characteristics of this device are equivalent to those of a liquid crystal display device in which the birefringence mode with the vertical orientation is divided into two.
That is, in actuality, the liquid crystal display device 100 disclosed in Non-patent literature 1 is inferior to a publicly-known liquid crystal display device of a four-domain type in the birefringence mode with the vertical orientation, more specifically, inferior to a device in the IPS mode, in its viewing angle characteristic.
However, in the IPS mode, each pixel is not divided into domains, so that it is difficult to obtain a sufficiently-wide viewing angle. In addition, the IPS mode has the drawback of coloring (color shifts) occurring depending on the direction of view. Specifically, a display in the IPS mode is colored yellow in a given direction of view and is colored blue in another direction of view. Therefore, the display quality is low in the IPS mode.
In view of this, a super in-phase switching (S-IPS) mode in which the IPS mode is improved to suppress coloring was proposed (see, for example, Patent literature 2.) As shown in FIG. 24, for example, a liquid crystal display device 100 in the S-IPS mode has a herringbone electrode structure. For example, the liquid crystal display device 100 includes: scanning lines 103 and signal lines 104 formed in a lattice pattern; TFTs 106 provided at respective intersections of the scanning lines 103 and the signal lines 104; comb-like pixel electrodes 101 connected to the TFTs 106 and serving as first electrodes; and comb-like common electrodes 102 serving as second electrodes and each formed between two adjoining pixel electrodes 101. The base ends of the common electrodes 102 are connected to a common line 105 that extends in parallel with the scanning lines 103 and passes through the center of each pixel.
The above-described herringbone electrode structure is formed by the pixel electrodes 101 and the common electrodes 102. A first display region 111 and a second display region 112 are formed at both sides of the common line 105 and serve as two domains. The initial orientation of liquid crystal molecules in the liquid crystal display device 100 is defined to be in the direction A parallel to the signal lines 104 by using an alignment film subjected to a rubbing process, as shown in FIG. 24.
When a voltage is applied across the pixel electrodes 101 and the common electrodes 102, electric fields are generated in a direction orthogonal to the length direction of the pixel electrodes 101 and the common electrodes 102. Accordingly, liquid crystal molecules 108a in the first display region 111 and liquid crystal molecules 108b in the second display region 112 rotate in different directions such that the directions of the liquid crystal molecules 108a and 108b approach the respective directions of the electric fields. This process is intended to obtain a wide viewing angle and to suppress coloring by making yellow coloring and blue coloring occur in the same direction to compensate for color shifts occurring depending on the direction of view.
Patent literature 1: Japanese Patent Publication No. 10-161128
Patent literature 2: Japanese Patent Publication No. 10-148826
Non-patent literature 3: IDW '99 p. 129 (International Display Workshop '99)